First of all, a process for continuous casting is outlined with reference to FIG. 1 which is a schematic diagram showing the upper part of a continuous casting machine into which molten steel is poured.
A problem involved in the conventional process of continuous casting of ultra low carbon aluminum killed steel is the clogging of the immersion nozzle 1 with Al.sub.2 O.sub.3 sticking thereto. Common practice to prevent the clogging is to blow an argon gas into the immersion nozzle 1 from the upper nozzle 2 or sliding nozzle 3. A disadvantage of this practice is that the argon gas becomes bubbles which are entrapped in the solidified shell during the step of continuous casting. The entrapped bubbles expand when heated during the step of annealing after rolling, swelling the surface of a cold rolled sheet.
There are three methods for preventing the clogging of the immersion nozzle without blowing an argon gas. They involve the addition of calcium to the molten steel being cast so that calcium changes Al.sub.2 O.sub.3 into a composite compound of CaO--Al.sub.2 O.sub.3 having a lower melting point, as disclosed in Japanese Patent Laid-open Nos. 99761/1989 (1), 276756/1986 (2), and 1457/1986 (3). According to the first disclosure, the tundish is provided with a refractory cylinder within 1 meter from the center of the tundish nozzle, with the lower end thereof immersion in the molten steel, and calcium is thrown into the cylinder in an amount equal to 5-20 ppm of the molten steel passing through the tundish nozzle. According to the second disclosure, calcium or a calcium alloy is added to the melt of aluminum killed steel containing less than 0.015 wt % of carbon, such that metallic calcium in an amount of 2-40 ppm remains to form CaO--Al.sub.2 O.sub.3 compounds in the steel. According to the third disclosure, an aluminum killed steel or aluminum-silicon killed steel containing more than 0.05 wt % titanium and more than 0.01 wt % aluminum is continuously cast after the composition has been adjusted such that the molten steel in the tundish contains 0.001-0.005 wt % calcium.
All the methods in the above-mentioned three disclosures have the following disadvantages.
(a) The resulting cold rolled steel sheet is subject to rusting depending on the chemical composition of steel (or the content of calcium and sulfur in steel) which determines the conditions of calcium addition. PA0 (b) The nozzle clogging may occur depending on the chemical composition of steel melt (such as content of calcium and oxygen in steel) or the continuous casting conditions, which prevents successive casting of many heats with one immersion nozzle.
Moreover, in the case where calcium is added but the blowing of argon gas into the immersion nozzle 1 is stopped, there exists no rising flow of molten steel induced by the buoyancy of gas in the mold 5. This results in the solidifying on the surface of the molten steel in the mold, which in turn leads to a high breakout ratio and the surface and inner defects of slabs cast. Also, in the case where the supply of argon gas is stopped, there exists no gas, which functions as a heat insulator, between the flow of molten steel and the inside of the immersion nozzle 1. This causes the molten steel to solidify on the inside of the nozzle above the surface of the molten steel in the mold. This in turn causes the nozzle clogging with solidified steel 6.
Incidentally, the ultra low carbon aluminum killed steel in the present invention denotes a steel which contains, in the steel melting step, less than 30 ppm of carbon and less than 40 ppm of oxygen (as the result of deoxidization mostly by aluminum).